1. Field of the Invention
This invention relates generally to tools that are used for drilling water and hydrocarbon wells and, more particularly, to a drill bit sub that generates a pulsating jet flow in drilling fluid circulated through a conventional drill bit. The drill bit sub of the invention can be used with either drill pipe or coiled tubing. Use of the subject drill bit sub to achieve greater reaches during horizontal drilling is also disclosed.
2. Description of Related Art
It has been suggested in the prior art to use acoustic energy for stimulating producing wells. A fluidic oscillator may be used to create pressure fluctuations to induce stress in the walls of the perforation tunnel, thereby increasing production and cleaning perforations as disclosed in U.S. Pat. Nos. 5,135,0531 and 5,228,508 issued to Facteau. The pressure fluctuations of the Facteau tool are generated from an oscillation chamber with two outlet ports. A similar fluidic oscillation chamber with dual outlet ports is disclosed in U.S. Pat. No. 5,165,438 also issued to Facteau. Another stimulation tool using acoustic energy is disclosed in U.S. Pat. No. 3,520,362 issued to Galle. Although the above recited tools seemed feasible, there exists a practical difficulty of delivering sufficient acoustic power to the producing formation for the desired stimulation and/or to the area to be cleaned.
IADC/SPE paper 27468 teaches the use of Helmholtz oscillator theory for generating a pulsating jet flow in drill bits. The pulsating jet flow is said to produce much higher instantaneous jet velocity and impact pressure than experienced with non-pulsating flow, resulting in significant improvements in hole cleaning and drilling rate. Pulsed high pressure water jets are known to have advantages over continuous jet streams for use in cutting materials, especially brittle materials. By exerting an alternating load on materials, pulsed jets can produce not only extremely high momentary pressures (i.e. water hammer effect) in the materials, but also absolute tensile stress, which gives rise to unloading destruction of brittle materials, through reflection of the stress waves. Pulsating jet flow is also known to reduce the hydraulic hold-down effect, thereby increasing the cutting and cleaning action of the jet. IADC/SPE 27468 discloses a new nozzle that generates the pulsating jet flow by a self-oscillation of fluid in the oscillation chamber. A xe2x80x9cbump surfacexe2x80x9d at the bottom of the oscillation chamber, shown for example in FIG. 3, is said to be a key factor in the quality of the pulsating jet flow. The article concludes that the nozzle shape, manufacturing technique, and the bit structure may be further studied to improve the pulsating jet flow quality.
A discussion of design parameters for self-excited oscillation jet nozzles is included in a paper entitled xe2x80x9cNozzle Device for the Self-Excited Oscillation of a Jetxe2x80x9d presented as Paper 19 at the 8th International Symposium on Jet Cutting Technology held in Durham, England, Sep. 9-11, 1986 and available from BHRA, the Fluid Engineering Centre, Cranfield, Bedford MK430AJ, England.
A drill bit sub is needed, however, that can create a self-excited, pulsating flow of drilling fluid in an oscillation chamber above a conventional drill bit of the type typically employed for drilling water, oil and gas wells using either a standard drilling string or coiled tubing. The desired drill bit sub should be useable with cone, blade or PDC bits having single or multiple ports.
The present invention comprises a self-excited drill bit sub that creates a pulsating flow of drilling fluid utilizing Helmholtz oscillation theory prior to entering the drill bit. The pulsating stream is caused by vortices which are created inside the tool by structural features not disclosed in the prior art. The vortices create pressure pulses as they leave the sub and travel into and through the ports in the drill bit. The cyclic pressure pulses facilitate cutting and removal of cuttings from the interface between the drill bit and the formation. In contrast to drill bits using jet nozzles that generate pulsed flow, the drill bit sub of the invention provides a longer flow path for the pulsed flow prior to exiting the bit, providing a more steady flow through the bit, and causing less erosion of the nozzles. Use of the drill bit sub disclosed herein also insures that all bit nozzles are pulsing in unison, and reduces the likelihood of forming a destructive standing wave when rotating while pulsating during drilling.
The drill bit sub of the invention desirably includes an elongated tubular first member adapted on an upper end for connection to a running string. The first member includes an upper portion with a central bore open to a top of the tool, and a lower portion having a cylindrical shaped cavity open to a bottom surface of the first member. The cylindrical cavity is internally threaded in a lower portion and has an internal diameter larger than the diameter of the central bore of the upper portion. The cylindrical cavity has an interior wall height of the unthreaded portion that is less than the diameter of the cylindrical cavity. The central bore of the first member is open to the cylindrical cavity for delivering drilling fluid supplied through the running string.
The drill bit sub further includes a second elongated tubular member having an upwardly directed, male threaded end and a central bore having a diameter larger than the diameter of the central bore of the first member but less than the diameter of the cylindrical cavity of the first member. Importantly, the top end surface of the second tubular member comprises a truncated annular conical projection having a blunt annular horizontal surface adjacent to the bore, a conical shoulder extending downward and radially outward from the blunt surface, and another annular horizontal surface extending radially outward between the base of the conical shoulder and the threaded edge of the male end. The male threads on the top of the second tubular member are receivable in the internally threaded lower portion of the first member. The second tubular member also preferably includes a downwardly facing, female threaded box end adapted to receive the male threaded pin end of a drill bit so that, when connected, the bit sub discharges pulsating drilling fluid through the bit exit ports and against the formation. When the drill bit sub of the invention is assembled with the male end of the second member threaded into the lower female end of the first member, an internal oscillation chamber is formed.
According to another preferred embodiment of the invention, a drill bit sub is provided that comprises an oscillation chamber disposed above a drill bit, the drill bit sub having at least two radially and axially spaced, annular impingement surfaces interconnected by an inclined annular surface. The oscillation chamber is preferably coaxially aligned and in fluid communication with upper and tubular bores that each have diameters less than the diameter of the chamber. According to a particularly preferred embodiment of the invention, the oscillation chamber has a diameter from about three to about five times the diameter of the upper tubular bore and a height between 1.6 and 5.6 times the diameter of the upper tubular bore. The diameter of the lower tubular bore is preferably about 1.3 times the diameter of the upper tubular bore.
In accordance with the present invention, the subject drill bit sub is easily fabricated and assembled, has no moving parts, and significantly increases the drilling rate as compared to a drill string using the same bit without the bit sub of the invention. In drilling, the subject drill bit sub achieves a higher penetration rate due to the higher impact pressure created from pulsing the jet stream and the reduction of the xe2x80x9chydraulic hold-down effectxe2x80x9d on the cuttings that is caused by conventional straight jets. The drill bit sub of the invention not only aids in the break-up of hard, brittle material but also aids in the cleaning of the bit and removal of debris from the hole bottom. The ability to keep the bit clean and the hole bottom free of debris causes the drill bit/hole bottom contact to be greatly increased, thereby increasing the bit penetration rate. Use of the subject drill bit sub requires no other changes in drilling procedure.
Some of the advantages of locating the oscillator of the invention in a separate drill bit sub above the drill bit are:
the sub can be made in various sizes to adapt to any size bit and collar combinations;
there is ample space to machine the oscillator directly into the collar;
the porting can be large enough to eliminate the xe2x80x9cwashingxe2x80x9d effect due to high pressures and abrasive material;
by using no moveable parts, the oscillator will operate at its maximum efficiency;
xe2x80x9cpluggingxe2x80x9d of the ports is virtually eliminated due to the port sizing; and
the drill bit sub can be machined from 4140 heat treated and hardened to s 2832 Rockwell C hardness, which is the same hardness as material used to make drill collars.